An inkjet printing system is characterized in that recording of images or characters is performed by attaching liquid microdroplets of an ink composition to a substrate to be printed on, and thereby printing plate is not used in the process of printing. As another printing system which does not use any printing plate, an electrophotographic system is also well known. However, in terms of equipment cost, running cost, printing speed and the like, the inkjet printing system is considered to be superior. For such reasons, there has been an increase in the market demand for on-demand printing in recent years, and the demand is even further expanding.
Compared to an ink composition used in off-set or gravure printing, the ink composition used in inkjet printing is required to have strict viscosity control. This is because the liquid droplet amount at discharging varies in accordance with a change in the viscosity of the ink composition, and as a result, the image quality of a printed material varies. Further, as the inkjet printing system is used all over the world in recent years, an ink composition having as little quality variation as possible like viscosity, that is, excellent stability over time, is required.
Meanwhile, accompanying the performance improvement of an inkjet head in recent years, a shift toward a small lot printing in an existing printing industry draws attention. In the printing industry, productivity is important, and the multi pass system used in a signature field cannot provide the required productivity. For such reasons, in order to have the productivity that is not obtained by the multi pass system, most of the inkjet printing used in the printing industry uses a single pass system having fast printing speed. Compared to the multi pass system, nozzle missing is clearly reflected to the image quality in the single pass system, and thus the ink is required to have an excellent discharge property.
Further, most of the substrates used in a signature industry are a vinyl chloride sheet. However, in the printing industry, various kinds of substrates such as paper-based or film-based substrates exist. Since the adhesiveness or spreading property of the ink varies depending on the types of substrates, the ink is required to have compatibility with various substrates. Further, to cope with fast printing speed and printing on various substrates, an ink with fast curing speed and excellent versatility in terms of adhesiveness onto a substrate is required. From this point of view, an active energy ray-curable ink is preferred most.
However, compared to other types of an inkjet ink composition, it is known to be difficult to control the viscosity or stability over time of the active energy ray-curable inkjet ink composition. That is because, the main component of the active energy ray-curable inkjet ink composition is a polymerizable monomer, and due to a polymerization initiating component generated in a tiny amount during transport or storage, the polymerization reaction of the polymerizable monomer progresses.
In other words, for the development of an active energy ray-curable inkjet ink composition, it is important to have a product quality such as curing property (productivity), discharge property, storage stability, or compatibility with various substrates as advantages.
Until now, various determinations have been made to solve the aforementioned problems. For example, Patent Literature 1 discloses printing ink having low viscosity and good discharge property containing 2-(2-vinyloxyethoxy) ethyl acrylate. However, it is difficult to exhibit a good curing property. In Patent Document 2, N-vinyl lactams are used as a polymerizable monomer and improvement of storage stability is obtained by adding an anti-oxidant or a polymerization inhibitor. However, the ink composition described in Patent Document 2 shows insufficient strength or resistance of a printed material to be obtained by the active energy ray-curable inkjet printing system. Further, although Patent Document 3 discloses a printing method using a single pass system, an excellent curing property is not easily exhibited.
As described above, although various determinations have been made regarding an active energy ray-curable inkjet ink composition, it is a present state that an ink composition satisfying all of the curing property, discharge property, storage stability, and compatibility with various substrates is not obtained yet.
Meanwhile, the active energy ray-curable inkjet ink is known to have better drying property compared to a solvent type ink. For such reasons, the active energy ray-curable inkjet ink is installed in a high speed printing type signage printer or, from the viewpoint of having excellent adhesiveness to a substrate, it is installed in a flatbed type printer having compatibility with various substrates, and a development of an ink blend formulation for each application has been made.
With those printers, printing with large size, thick film, and high density could be achieved by scanning a head. In addition, a head technology for allowing extrusion of liquid microdroplets by high frequency based on recent developments in head technology has been established. With realization of those techniques, there is now a high possibility that the inkjet printing which has been remained unfavorable in terms of productivity and image quality becomes alternative of the existing printing systems, in conjunction with the merit of digitalization. However, for achieving those technological improvements, development of ink having better higher frequency suitability than before and also with low viscosity and high sensitivity is needed. Among them, as a specification required for ink, the low viscosity can contribute to an improvement of shot accuracy and it is important to obtain an image with high definition.
In Patent Document 4, a UV curable-type inkjet ink with low viscosity is suggested. With the disclosed ink, at least low viscosity can be achieved. However, when printing is made with high frequency of 20 kHz, missing printing occurs in accordance with the printing for a long period of time. For such reasons, it is not suitable for printing at practical level, and thus further improvements are needed. Further, a technological handout, that is, Non Patent Document 1, is made available by The Lubrizol Corporation in which determinations of a pigment dispersion having excellent fluidity are made by dispersing several pigments with use of a corresponding dispersing resin. The pigment dispersion using the dispersing resin exhibits excellent fluidity, but for achieving desired ink with low viscosity and high sensitivity, developments of an additional formulation of ink are needed.